Silver halide emulsions used in silver halide photographic materials are usually subjected to chemical sensitization using a sulfur sensitizer, a selenium sensitizer, a reduction sensitizer, a noble metal sensitizer, etc., either alone or in combination, for the purpose of obtaining a desired sensitivity and gradation. Among others, sulfur sensitizers, selenium sensitizers and noble metal sensitizers are important.
Further, for the purpose of attaining excellent color reproduction, silver halide emulsions are spectrally sensitized with sensitizing dyes so as to exhibit sensitivity to light of longer wavelengths to which silver halides are by nature substantially insensitive.
With the recent demand for increasing sensitivity of silver halide emulsions, particularly in the wavelength region for which spectral sensitization is performed, it has been attempted to increase the amount of the sensitizing dye to be added to the silver halide emulsion to increase the light absorption.
A spectral sensitization sensitivity S.lambda. (at a wavelength .lambda.) obtained by addition of a sensitizing dye can be determined according to the equation: ##EQU1## wherein S.degree.400 and S400 represent the photographic sensitivity of the spectrally non-sensitized emulsion and that of the spectrally sensitized emulsion, respectively, at a wavelength of 400 nm; .phi..sub.r represents a relative quantum efficiency; and A.lambda. and A400 represent percent absorption at a wavelength of .lambda. and 400 nm, respectively.
Addition of a large quantity of sensitizing dyes is favorable for increasing absorption but, at the same time, causes reduction of .phi..sub.r or reduction of S400/S.degree. 400 (generally called "desensitization of intrinsic sensitivity"), which ultimately results in reduction of photographic sensitivity.
Although various supersensitization techniques have been developed for improving .phi..sub.r or preventing desensitization, the inefficiency resulting from an approach of a saturated adsorption on silver halide grains cannot be sufficiently suppressed by these techniques.
Simson et al. report that inherent desensitization does not occur when a sensitizing dye is adsorbed onto the surface of internal latent image type emulsion grains whose core has been chemically sensitized, as described in J. W. Simson & W. S. Gavgh, Photographic Science Engineering, Vol. 19, 339 (1975). However, since the emulsion of this type exhibits internal sensitivity, no image appears when developed with a surface developer. Besides, a color developer used for color photographic materials is not applicable to the internal latent image type emulsion because of its low solubility. All the other conventional developers have insufficient solubility to be applied to the internal latent image type emulsion.
It has also been proposed to use a shallow internal latent image type emulsion which forms a latent image in a very shallow portion beneath the grain surface. However, if silver halide grains have a suitable shell thickness to be developed with a developer having ordinary solubility, desensitization would be likely or development would be considerably retarded.
A chemical sensitization technique is desired which provides a high sensitivity silver halide emulsion without causing reduction of inherent sensitivity due to a dye, as is encountered in using a developer having low solubility.
If chemical sensitization nuclei, i.e., positions where a latent image is to be formed, can be formed on the surface of silver halide grains, unlike the method of Simson et al., apart from most of adsorbed dye particles, the reduction of inherent sensitivity due to the dye should be suppressed even when the silver halide is developed with a general developer of low solubility. The conventional techniques, including the method of Simson et al., rarely have referred to possibility of isolating latent image specks from an adsorbed dye as well as controllability of the position of the chemical sensitization nuclei where a latent image is to be formed. However, intentional formation of chemical sensitization nuclei at a limited position of the surface of silver halide grains without scatter would favor a silver halide emulsion with increased sensitivity. Accordingly, it has been keenly demanded to develop a method for highly controlling the position of chemical sensitization nuclei, and to produce a high sensitivity silver halide emulsion obtained thereby
There are some reports with respect to addition of dyes, such as methine dyes, to a silver halide emulsion during chemical sensitization.
Further, several cases have been known where a sensitizing dye is added to a silver halide emulsion at the beginning of chemical sensitization as described, e.g., in U.S. Pat. No. 4,435,501 and Japanese Patent Application No. 62-141112. However, these cases concern silver halide twins (tabular grains). JP-A-61-133941, JP-A-59-9153, JP-A-58-28738 and JP-A-62-7040 also refer to the addition of a sensitizing dye at the time of chemical sensitization. (The term "JP-A" as used herein refers to a "published unexamined Japanese patent application".)
Furthermore, Japanese Patent Application No. 61-311131 describes control of positions of development centers, i.e., positions of chemical sensitization, and particularly formation of development centers, i.e., chemical sensitization nuclei, on a (111) plane. Moreover, the dye is employed without being accurately evaluated for its adsorption selectivity, and halogen conversion is chiefly used here.
Japanese Patent Application No. 62-152330 teaches the use of a compound called a "CR compound" in order to form a development center on the top of octahedral or tetradecahedral normal crystals having a (111) plane, that is, on a plane other than the (111) plane.
In addition, it is also known to add a dye at the time of grain formation preceding chemical sensitization as disclosed, e.g., in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756 and 4,225,666, JP-A-60-196749, JP-A-61-103149 and JP-A-61-165751, and Research Disclosure, No. 19227, Vol. 192, 155 (1980). In most of these cases, the dye added exists in the system during the subsequent chemical sensitization.
Some chemical sensitizers which selectively sensitize a (100) plane instead of a (111) plane, and particularly sulfur sensitizers, are known in the art, as described in Research Disclosure, Nos. 17643 and 18716, J. Phys. Chem., Vol. 57, 725 (1953), U.S. Pat. Nos. 2,278,947 and 2,410,689, and JP-B-58-28568 (the term "JP-B" as used herein refers to an "examined Japanese patent publication").
Selective chemical sensitization is referred to in Journal of Photographic Science, Vol. 23, 249 (1975), describing that sodium thiosulfate chemically sensitizes a (111) plane selectively.